The Winter War (Finnish: talvisota, Swedish: vinterkriget, Russian: Зимняя война, tr. Zimnyaya voyna) was a military conflict between the Soviet Union and Finland. It began with a Soviet offensive on 30 November 1939—two months after the start of World War II and the Soviet invasion of Poland—and ended on 13 March 1940 with the Moscow Peace Treaty. The League of Nations deemed the attack illegal and expelled the Soviet Union from the League on 14 December 1939.

The Soviet forces had more than three times as many soldiers as the Finns, thirty times as many aircraft, and a hundred times as many tanks. The Red Army, however, had been crippled by Soviet leader Joseph Stalin's Great Purge of 1937, reducing the army's morale and efficiency shortly before the outbreak of the fighting. With more than 30,000 of its army officers executed or imprisoned, including most of those of the highest ranks, the Red Army in 1939 had many inexperienced senior officers. Because of these factors, and high commitment and morale in the Finnish forces, Finland was able to resist the Soviet invasion for far longer than the Soviets expected.